1. Field of The Invention
This invention relates to light modulating systems; and it relates more particularly to providing in such systems compensation for space-charge-induced drift in the characteristics of a solid state modulation device.
2. Description Of The Prior Art
Solid state light modulating devices, sometimes called electro-optic modulators, are well known in the art. Also well known is the fact that the application of an electric drive signal to an electro-optical modulator causes, over time, the build up of space charge within the device which has the effect of partially offsetting the modulating effect of the drive voltage. That is, the growing offset causes the light transmission versus applied drive voltage characteristic of the modulator to drift. Such drift quickly becomes intolerable (e.g., in a system for exposing photographic film) because, without space charge compensation, it renders impossible the faithful reproduction of a scene that is to be reproduced.
One solution to the space charge problem has been to introduce a periodic reversal of polarity in the modulator drive voltage to prevent the build up of space charge. One example of this type of system is in the electro-optic line printer of the U.S. Pat. No. 4,369,457 to R. A. Sprague. However, sometimes spurious factors such as unbalanced duty cycle of data representing characters being printed can allow space charge to build up, in spite of the reversals, over a relatively long period of time.
Another approach to resolving the space charge problem is to employ a closed continuous feedback loop in which a portion of the modulated light transmitted by the modulator is diverted to a photodiode which converts the light intensity level to an electric signal. That signal is then used on a continuous basis to modify the modulating electrical drive signal to the modulator. Two U.S. Pat. Nos. related to this type of system are 4,631,551 and 4,667,256, both to A. B. Vergona. Although the systems disclosed in these two patents to Vergona have been found to work in their intended manner, a system employing a feedback loop of this general type suffers from two potential problems. One problem is that some modulator systems operate in a low light level range in which diversion of sufficient light to be useful in a continuous feedback network cannot be tolerated. Another problem is that the transmission characteristic of at least some solid state modulators is a sine squared function of the form: EQU T=A+B sin.sup.2 (CV.sup.2)
where T is the transmission; A, B, and C are constants related to modulator material geometry and space charge state; and V is the modulating voltage signal. A small increase in voltage can, if not carefully controlled, cause a substantial change in the frequency term, a shift in modulator operating point to a different cycle of the characteristic, and a transmission change that may not be easily predictable. Thus, continuous feedback operation can throw the device into an unstable condition unless rather sophisticated and costly circuit design precautions are adopted.